Abstract

We investigate the comb linewidths of self-referenced, fiber-laser-based frequency combs by measuring the heterodyne beat signal between two independent frequency combs that are phase locked to a common cw optical reference. We demonstrate that the optical comb lines can exhibit instrument-limited, subhertz relative linewidths across the comb spectra from 1200to1720nm with a residual integrated optical phase jitter of ∼1rad in a 60mHzto500kHz bandwidth. The projected relative pulse timing jitter is ∼1fs. This performance approaches that of Ti:sapphire frequency combs.

Figures (4)

(a) Schematic of the experimental setup. FC, fiber coupler. Inset, spectra of comb 1 (gray) and comb 2 (black). Arrows indicate the location of the different optical bandpass filter settings for the heterodyne measurements. (b) Comb 1 (top) and comb 2 (bottom), along with the cw reference laser at 1550nm(194THz). The circled pairs contribute to the beat note at 8MHz. (The number of contributing pairs is ∼400–4000, depending on the filter bandwidth.)

(a) In-loop phase noise spectral densities for the lock between the cw reference laser and comb 1 (gray) and comb 2 (black). The integrated phase noise of 0.94 and 0.35rad, respectively, is dominated by the servo bumps at high Fourier frequencies. Extrapolation to the Nyquist frequencies of 92.5 and 25MHz for the two combs gives integrated phase noises of 0.53 and 1.21rad. (b) In-loop phase noise for fceo for comb 1 (gray) and comb 2 (black). The integrated phase noises are 1.5 and 1.0rad, respectively.

Spectra of the coherent peak across the comb and on fbeat,i between the combs and the cw reference laser (resolution bandwidth (RBW) of 0.3Hz, acquisition time 48s). Inset, rf spectra on a 1MHz span and log scale for the heterodyne beat at 1720nm(RBW=3kHz).